On Ubuntu 20.04, I use my own non-standard theme (it’s a slightly modified version of Radiance theme, because Radiance had some small things that really annoyed me, so I got rid of them). I called it “My Theme” and installed in “/usr/share/themes/My Theme” ). It works OK for regular GNOME applications, but not for snaps.
At first when I tried it, snap applications (in particular Snap Store, because I don’t have any other installed yet) had completely transparent background when non-standard theme was selected. I asked about it here: https://askubuntu.com/questions/1245851/ubuntu-20-04-gnome-flashback-snap-shop-transparent-on-non-default-themes
I managed to fix it by installing “overlay” snap and remounting “/snap/gnome-3-34-1804/33” (that was the current version then) read/write using it. Then I copied “My Theme” to usr/share/themes inside this snap and it worked.
However with “/snap/gnome-3-34-1804/36” (which seems to be recent version now), this method no longer works - the background is no more transparent, but snap applications use the default Adwaita theme and not “My Theme”.
I also tried to remount “/snap/gtk-common-themes/1506” r/w and copy the appropriate parts of “My Theme” into share/gtk-2.0 and share/themes inside this snap, but this doesn’t work too.
My question is: what to modify inside /snap to make snap applications use my theme?
Hi,
coming from there Adapta-gtk-theme-colorpack I’d say : build and publish your own snap of your modified Radiance theme ?
Mmm, yes it sounds a bit too much. I’d have liked to be able to modify locally the gtk themes bundled in /snap/gtk-common-themes but is it only possible ?
Well… fixed! 
It seems that now I should copy my theme not into /snap/gnome-3-34-… but /snap/gnome/3-28-… and it works again!
The “snap connections” command mentioned in the thread you linked gave me a hint - “gnome-3-28-1804” was indicated as connected while “gnome-3-34-1804” was not.